Electric motors are increasingly becoming an important part of the propulsion of an unmanned aerial vehicle (UAV). Due to the load-carrying limitation of the UAV, the UAV motors need to reach a power density as high as possible, meanwhile ensuring absolute system reliability. High power density may often need higher density of current in windings of the motors or bring larger iron loss, which all may cause more heat. High system reliability may require an operating temperature of the system to be as low as possible, and the operating temperature is never allowed to exceed a safe temperature threshold. Otherwise, the motor may be damaged due to overheat, or even be burnt. In the past, in order to achieve one of the two such objectives, the other one is often sacrificed. For example, the system reliability may be achieved by increasing a capacity of a motor, which often sacrifices the power density.
Some existing protective fractions against overheating are simply to turn off the whole motor, causing failure of a part or all of the systems in the UAV, or even UAV crash in severe conditions.
Current technologies often uses two basic approaches: (1) sacrificing one of the power density and high reliability while optimizing the other. For example, a high power density is achieved by scarifying a part of the reliability. Alternatively, for high reliability, a motor with larger capacity is used or the power of an existing motor is intentionally limited, scarifying high power density. (2) When the motor temperature reaches a certain upper limit, the motor is simply turned off for protection. However, in UAV applications, turning off the motor may often bring unacceptable consequences.